The present application relates generally to image forming devices, and more particularly to toner density tests in image forming devices.
Image forming devices optically form a latent image on a photoconductive member, and develop the image by applying toner. The toner is then transferred—either directly or indirectly—to a media sheet where it is deposited and fixed, such as by thermal fusion. In particular, it is known to successively transfer developed color-plane images from one or more photoconductive members to an intermediate transfer member, and subsequently transfer the developed image to a media sheet for fixation thereon. Examples of an image forming device utilizing an intermediate transfer member are the Model C750 and C752 devices from Lexmark International, Inc. Alternatively, it is known to direct a single media sheet past one or more photoconductive members, each of which successively transfers a developed color-plane image directly to the media sheet. An example of a direct transfer device includes Model C534, also from Lexmark International, Inc.
A problem common to image forming devices, regardless of their configuration, is toner density control. Numerous methodologies are known in the art for measuring the density of toner disposed on an intermediate transfer member or media sheet. Many of these include the steps of transferring developed images comprising test patterns of various forms to a test surface and detecting the developed images on the surface, e.g., detecting the presence of toner on the surface. One way to detect the toner is with optical density sensors.
Optical density sensors are well known in the art. An optical density sensor measures the presence, and may determine an amount (e.g., in gm/cm2), of toner on a surface. This measurement may be performed indirectly, such as by sensing the differing optical properties of the surface and of toner deposited on the surface. One way to sense these properties is to illuminate the surface with a light source and sensing and measuring the resulting reflections. The sensed light is translated to toner density data through calibration procedures, as well known in the art.
The sensor outputs a data stream proportional to the sensed light. When the data stream is the result of reflections by a periodic group of toner patterns, the data stream includes periodic areas of data corresponding to the reflections caused by the test patterns interspersed with periodic areas of data corresponding to reflections caused by the intermediate transfer member or media sheet. To process the data associated with the toner test patterns, data collection windows are aligned with the toner test pattern data, and a controller processes the data contained within the data collection windows. It will be appreciated that the accuracy of the resulting toner density measurements directly relates to how accurately the data collection windows align with the actual test pattern data. The alignment may be compromised by an offset error caused by various mechanical tolerances, such as vibrations, velocity errors, sensor location errors, etc. Further, the alignment may be compromised by magnification errors, such as printhead magnification errors, that cause irregular spacing in the test pattern data relative to the regularly spaced toner test patterns. The alignment may also be complicated by the size of the toner test patterns. For example, the current trend is to reduce the size of the toner test patterns to increase productivity and to reduce the calibration time of the image forming device. However, smaller toner test patterns amplify the effects of the mechanical and magnification errors, which may cause processing errors that produce inaccurate toner density measurements.
FIG. 1 shows one prior art solution, which is described in U.S. Pat. No. 6,044,234. The illustrated process involves computing an offset error between a centerline for a nominal expected correction test pattern and a centerline for the actual correction test pattern. The position of the data collection window for subsequent toner test patterns is adjusted based on the computed offset error. In so doing, the prior art solution may correct for mechanical errors, but does not address magnification errors.